Sex hormones are steroids synthesized by gonads through multiple biochemical mechanisms. In mammalian systems, there are six families of steroid hormones that can be classified on both a structural and a biological (hormonal) basis. They are the estrogens and progestins (female sex steroids), androgens (male sex hormones), mineralocorticoids (aldosterone), glucocorticoids (cortisol) and vitamin D. They are all derived from cholesterol. The parent ring structure for cholesterol is the fully saturated ring structure cholestane. Steroid hormones are fat-soluble so they can enter all body cells because lipid cellular barriers are not a barrier for them. Regulatory cognitive production of sex hormones is a complex process, and all mechanisms take place through the hypothalamus-pituitary-ovarian linkages. The hormone biosynthesis determines the sex cycle of the animals, but also depends on the sex cycle of the same individuals, therefore the biosynthesis in pregnancy is different from the one in the reproductive age. Pregnancy is characterized by a massive increase in the production of both progesterone and estrogen. The increase in progesterone and estradiol production occurs only in the placenta. Male sex hormones (androgens) are steroids with 19 C-atoms, and female with 18 C-atoms (estrogens) and 21 C-atoms (progesterone). Androgens are synthesized in Leydig's cells from cholesterol, progesterone in the corpus luteum, and estrogen in the ovaries and corpus luteum, while less amounts are produced in the testicles and the adrenal gland. In the mitochondria, cholesterol is converted into pregnenolone, the precursor steroid required for synthesis of all steroid hormones. The reactions catalyze enzymes such as: P450scc, StAR, 5α-reductase. The most active estrogen is 17β-estradiol, and testosterone 5α-dihydrotestosterone (5α-DHT). Progesterone, testosterone and estradiol have multiple roles in sex cycles, and their function will depend on the physiological synthesis of the hormones themselves.